Apparatus and method for vaporizing molten metal

ABSTRACT

APPARATUS AND METHOD FOR VAPORIZING MOLTEN METAL WITHOUT ENTRAINMENT OF LIQUID DROPLETS INCLUDES PROVIDING A POOL OF MOLTEN METAL IN A CHAMBER HAVING PLURAL PARALLEL ELONGATED GROOVES FORMING A SCALLOPED BOTTOM PORTION, THE BOTTOM AND SIDES OF EACH GROOVE BEING ROUNDED. THE TOP OF THE POOL OF MOLTEN METAL IS MAINTAINED AT A LEVEL JUST ABOVE OR BELOW THE TOPS OF THE GROOVES. THE METAL IN THE GROOVES IS HEATED TO GENERATE VAPOR. TO REMOVE ANY DROPLETS FROM THE METAL VAPOR IT IS PASSED THROUGH A BAFFLE DEFINING A TORTOUS PATH AND THEN THROUGH A FILTER MADE OF A FELT MATERIAL THAT IS NON-REACTIVE WITH THE METAL   VAPOR. WHEN THE METAL VAPOR IS ZINC, THE FELT MATERIAL IS GRAPHITE FELT. THE VAPORIZED METAL IS THEN REMOVED FROM THE CHAMBER AND DEPOSITED AS A COATING ON A SUBSTRATE.

pll 3, 1973 J, M ROBUN EVAL APPARATUS AND METHOD FOR VAPORIZING MOLTEN METAL Filed June 24, 1970 2 Sheets-Sheet l CCCCCCCCCCCCC INVENTOR NE Lm Q R .JG M UNA D mmm OFJ J u/BY i RNE ORMEY April 3, 1973 APPARATUS AND METHOD FOR VAPORIZING MOLTEN METAL Filed June 24, 1970 J. M. ROBLIN ETAL 2 Sheets-Sheet 2 3/ Z195 l l rfa f United States Patent O M U.S. Cl. 75-93 28 Claims ABSTRACT OF THE DISCLOSURE Apparatus and method for vaporizing molten metal without entrainment of liquid droplets includes providing a pool of molten metal in a chamber having plural parallel elongated grooves forming a scalloped bottom portion, the bottom and sides of each groove being rounded. The top of the pool of molten metal is maintained at a level just above or below the tops of the grooves. The metal in the grooves is heated to generate vapor. To remove any droplets from the metal vapor it is passed through a baille defining a tortuous path and then through a filter made of a felt material that is non-reactive with the metal vapor. When the metal vapor is zinc, the felt material is graphite felt. The vaporized metal is then removed from the chamber and deposited as a coating on a substrate.

BACKGROUND OF THE INVENTION This invention relates to vaporizing molten metal, particularly zinc, while preventing particles (liquid droplets, eg.) from being entrained in the vapor.

Vaporized zinc coatings on steel offer many advantages over conventional electroplated zinc coatings and hot dip galvanized coatings. A smooth, non-splattered coating can be produced by vapor deposition while coating thickness can be varied by controlling the zinc evaporation rate and/or the speed of a moving steel substrate. A one-side coating can be easily produced by properly shielding the strip. Composite coatings, e.g. ya thin aluminum coating over a thin zinc coating, can be produced by vapor deposition, the steel thus coated having excellent corrosion resistance. Production speed can be considerably faster for vapor deposition than for an electroplating line or a hot dip galvanizing line because the restrictions of current density and coating roll speed do not apply. There is no inherent limitation on the gage of the steel being coated, unlike galvanizing processes where line speed rnust be lowered as thickness and width of strip are increased, due to the amount of heat needed to be transferred to the substrate. Furthermore, the cost of vapor deposition is believed to be less than the cost of either electroplating or hot dip galvanizing.

Traditionally, metals have been evaporated from crucibles or chambers simply by heating the metal to a ternperature above its `vaporization point and then removing the vapor from the crucible or chamber, e.g., by vacuum methods. At a high rate of vaporization, solid or liquid particles may become entrained in the vapor. It is not necessary to distinguish between liquid and solid particles as both have the same deleterious effect when present in the form of splatter upon a substrate. When coating a fast-moving substrate with the vapor, the entrained particles will create splatter on the surface of the substrate. Several methods of avoiding the splatter problem have been tried. One involves locating a baille plate or other baille arrangement between the surface of the pool of molten metal to be evaporated and the area where the vapor is to be collected. Providing a tortuous path is somewhat effective in removing large inclusions from the vapor, but it has not been found effective in completely preventing splatter on the substrate to be coated.

3,725,045 Patented Apr. 3, 1973 ICC Hunt et al. U.S. Pat. No. 3,330,647, discloses the use of a grid disposed substantially at the upper surface of a pool of molten metal. According to the patent, the grid is wet by the molten metal in order to prevent explosive evaporation. Gimigliano et al. U.S. Pat. 3,458,347 teaches the use of a plurality of cylindrical projections extending upwardly from the bottom of a vaporization crucible. The projections, which are not wetted by the molten metal, are disclosed as providing a way of escape for the generated vapor without the presence of liquid particles in the generated vapor.

Neither Gimigliano et al. nor Hunt et al. appreciated that, in order to effectively prevent entrainment of particles in generated vapor, it is necessary to provide the crucible with Ia particularly formed vaporization area having means for preventing bubbles in the molten pool from growing beyond a critical size.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide apparatus and method for vaporizing molten metal without entrainment of particles in the metal vapor.

It is another object of the invention to deposit a metal vapor as a coating upon a substrate moving at high speed without splatter in the coating.

Still another object of the present invention is to provide for the vaporization of metal wherein the rate of vaporization is relatively independent of molten metal level in a crucible.

These and other objects of the present invention are achieved through the use of metal vaporizing apparatus in which a Crucible contains a pool of molten metal, the Crucible having plural elongated grooves forming a scalloped bottom portion in which the molten metal is disposed. The grooves are substantially parallel to each other and are rounded along their respective bottoms and sides, the distance across each groove beingless than the critical diameter of a vapor bubble within the molten metal. The grooved bottom portion of the chamber provides good heat transfer, and may be fabricated of a material that is not wetted by the molten metal to provide escape points for the vaporized metal. The grooved bottom configuration provides stable, controllable and rapid boiling of the metal. Graphite has been found suitable as a crucible material when zinc is the molten metal. The level of molten metal is maintained just slightly above or below the tops of the grooves in the bottom portion of the crucible.

Means are provided to remove any liquid inclusions from the vapor which may exist despite the provision of the grooves in the bottom ofthe crucible. The metal vapor first flows through a baille structure that defines a tortuous path to remove any large droplets from the vapor. Forming the baille structure are a rst layer of plural parallel elongated strips arranged so that spaces are left between adjacent strips, a second layer disposed above the first layer and of similar construction, the strips of the second layer being located above the respective spaces in the first layer, and a third layer disposed above the second layer and of similar construction, the strips of the third layer being located above the respective spaces of the second layer. Each strip may be curved about its longitudinal axis and is arranged such that it is concave relative to the bottom of the chamber.

Although the baille removes large size particles from the vapor and prevents them from leaving the chamber, a filter is also provided adjacent the baille to remove any smallerdroplets remaining in the vapor. The filter is of a felt material that is non-reactive with the metal vaporized; when zinc is vacuum evaporated, the felt material may comprise graphite felt. A screen positions the filter against the baffle.

By use of thev method and apparatus of the present invention, metal vapor can be deposited on a continuously moving substrate such as steel strip to form a splatterfree coating thereon.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional side view o-f representative apparatus in accordance with the present invention.

FIGS. 2, 3 and 4 are sectional views taken along the lines 2-2, 3-3 and 4--4 of FIG. 1.

DETAILED DESCRIPTION Referring to FIG. 1 of the drawings, there is shown metal vaporizing apparatus which is maintained under vacuum and which includes a crucible 12 in which molten metal is disposed to be vaporized. The crucible 12 is enclosed by an outer shell 22 fabricated of low carbon steel, for example. A wall within the crucible serves as a support for a baffle structure 30, a filter 32, and a screen 34 which together serve to remove liquid droplets from the generated vapor. Molten metal is added to the crucible by means of an inlet pipe 18. The crucible is typically fabricated of a heat conductive material which is non-reactive with the molten metal and which is alsonot wetted by the molten metal. Thus in the case of zinc metal which is to be vaporized, the crucible 12 may be made of graphite.

Disposed between the outer shell 22 and the crucible 12 are a plurality of heating elements 26 which provide heat for vaporizing the molten metal. The heating elements 26 may be of any suitable type, such as furnace heater tubes enclosing burning gas, resistance heating graphite rods or the like. The heating elements 26 may be disposed at both the top and bottom of the crucible so that sufficient heat is provided to vaporize the molten metal.

The bottom portion 12a of the crucible is grooved or scalloped as shown to provide for good heat transfer to the molten metal within the crucible as well as to permit the vaporization of the molten metal while minimizing liquid inclusio-ns in the vapor, all to be described below. Insulation designated may be included within the outer shell 22 to provide for proper retention of heat within the crucible. Refractory bricks 27 may also be included below the crucible 12.

Proper design of the bottom portion 12a of the crucible 12 is critical. In order to properly construct metal vaporizing apparatus it is first necessary to briefly consider the principles of boiling heat transfer. The amount of energy transferred from one body, i.e., the crucible 12, to another, i.e., the molten metal, is dependent upon the area of contact, A, the temperature difference, AT, and the heat transfer boiling coeicient, h, between the two bodies. When the values for AT are low and the temperature of the crucible, Tc, approaches the saturation temperature of the liquid, the heat transfer may be calculated from the equation:

The rate of heat, transfer, q, is directly proportional to the rate of molten metal evaporated. For higher values of AT, the heat transfer boiling coefficient is not constant, but is a complex function of AT whose form is dependent principally upon the existing boiling regime, which in turn is dependent upon the physical properties of the liquid and the boiling surface and upon the ambient pressure.

Overall heat transfer during boiling follows a so-called boiling curve. For a liquid at its saturation temperature, Ts, heat transfer boiling coeicient at first increases slowly with increasing chamber temperature, Tc. During this time heat transfer occurs chiefly by convection, evaporation occurring at the liquid surface. Subsequently, AT reaches a point where surface nucleation of bubbles occurs, provided that the molten metal wets the chamber surface and that suitable nucleation points exist on the chamber surface (the significance of these factors will be discussed below). The bubbles formed during surface boiling collapse in the pool of molten metal before reaching the surface, evaporation taking place simply by surface evaporation from the saturated liquid.

As AT continues to increase, nucleate boiling occurs. In that regime the bubbles are able to reach the surface of the molten metal and most of the evaporation then takes place. Extremely high heat transfer boiling coefficients are attained under these conditions. When all available bubble nucleation sites have been exhausted, AT presumably reaches a critical point, beyond which bubble agglomeration begins to occur and the boiling surface begins to be progressively covered by an insulating layer of vapor. This leads, at increasing chamber temperatures, to a decrease in heat transfer until the Leidenfrost point is reached, at which point the entire boiling surface is covered by the vapor barrier. Subsequently, a regime of stable film boiling is reached in which heat transfer occurs by the collapse and regeneration of the vapor layer. Boiling is extremely violent since bubbles are ejected from large areas of the boiling surface. During stable film boiling heat transfer is not particularly sensitive to AT so that an increase in chamber temperature has little effect on the evaporation rate. Since the boiling surface is not actually in contact with the liquid metal owing to the vapor film, there is no effect of surface structure or roughness. At sufficiently high chamber temperatures heat transfer again increases with chamber temperature up to the melting point of the boiling surface.

It is now important to consider the theory of nucleate boiling in order to understand its significance to Vaporizing apparatus design. Nucleate boiling involves the nucleation of vapor bubbles in a liquid at its saturation temperature for a given ambient pressure. Homogenous nucleation of bubbles within the liquid is difficult and if forced to occur will be accompanied by a measurable superheating of the liquid. It is therefore postulated that bubble nucleation takes place heterogeneously at discrete sites on the boiling surface; these sites may be considered as microcavities in the boiling surface.

Molten metal will flow into a microcavity provided that the liquid wets the containing portion of the chamber, vapor formed at the root of the cavity, where heat transfer is greatest, expands until the bubble attains a critical size, the radius of which may be calculated from the following equation:

where:

r=bubble radius a=surface tension of the liquid Pv=vapor pressure inside bubble Plzpressure in liquid.

Upon release of the bubble the cycle is repeated by the vapor remaning in the cavity. Both the ease of obtaining nucleate boiling and the peak heat transfer rate are related to the number, size and distribution of such cavities. It should be noted though that nucleate boiling cannot occur if there is not adequate wetting of the chamber bottom by the molten metal. Under non-wetting conditions liquid will not flow into microcavities and allow the nucleation of bubbles. Hence, boiling must take place from vapor film formed at the liquid-chamber interface. This is substantially the same as normal film boiling, where there is an excessive amount of splatter in the generated vapor. Heat transfer will be low and relatively insensitive to AT and heat transfer at high chamber temperatures will take place by radiation through the vapor barrier.

The amount of wetting between a boiling liquid and the heating chamber greatly affects both the rate of boiling and the type of boiling, both of which in turn influence the amount of liquid droplets in the vapor. More liquid entrainment may be expected when the boiling liquid does not wet the chamber than when the chamber is wetted In the non-wetting case, because of the low attraction between the liquid and the chamber, a very thin, perhaps even monomolecular layer of vapor will form between the liquid and the chamber. Very slight superheating will cause vaporization of some of the liquid such that a vapor'bubble is formed which ascends through the liquid to enter the vapor stream. As it ascends the bubble tends to drag the liquid along with it, although this tendency is opposed by the weight of the liquid, and by the adhesion of wetting force between the liquid and the chamber. Thus, the less the tendency of the liquid to wet" the chamber, the more easily the liquid can be dragged out of the pool of molten metal by the vapor bubble. If the bubble breaks through the surface of the liquid, droplets will be generated which eventually will result in splatter on the substrate to be coated.

Providing a scalloped crucible bottom portion 12a rcsults in a reduction of liquid droplets in the vapor by reducing the size of the bubbles formed. This reduction is effected by limiting the size of the molten metal pool surrounding each bubble nucleus. With a smaller supply of liquid available the ultimate size reached by the individual bubble is smaller and fewer droplets are then generated when the small bubbles pass through the surface of the molten metal. Therefore, in the bottom portion 12a of the crucible there are formed plural elongated grooves 28 in which the molten metal is disposed for vaporization. As best seen in FIGS. 2 and 4, the grooves 28 are substantially parallel to each other and are interconnected by means of slots 31 arranged transversely to the orientation of the grooves. Each groove 28 is rounded along it sbottorn 28a and its sides 28b. Rounding each groove 28 results in a reduction of violent boiling as the metal vapor bubbles are permitted upon heating to slip along the bottoms 28a and the sides 28b of the grooves. Although other configurations might be utilized for the grooves 28, rounded grooves have been found to be most successful in reducing splatter. The grooves 28 are formed such that the distance across each groove is less than the critical diameter of vapor bubbles of the particular molten metal. Zinc vapor bubbles, for example, are thought to have a critical diameter of about 3%" and form from a surface of about twice the bubble diameter. Thus, where zinc is the molten metal to be vaporized, the grooves 28 should not be greater than about 2 in. in diameter and 1 in. in depth. If there is a restricted area which does not accommodate this critical diameter of the bubbles, the bubbles will not -fully form and violent boiling will not take place, thus preventing inclusion of liquid droplets in the vapor since there are no large bubbles to break. It should be understood that although other metals may be vaporized using the present invention, zinc coatings are common in the steel industry and are thus used as an example for the purpose of this application.

Pour critical parameters appear to determine whether liquid droplets will be present in vapor. The temperature of the molten metal should not be such as to cause vigorous boiling, e.g., for liquid zinc it should not exceed l275 F. (with scalloped bottom; without a scalloped bottom the maximum temperature would be about l100 F.). The residual gas pressure on the molten metal consequently should be less than about 200 microns. The depth of the molten metal should be -kept such that the tops 28e of the grooved crucible bottom portion 12a are slightly below or above the surface of the liquid. If the tops 28e are kept below the surface of the molten metal at all times, the molten metal-grooved surface interface area will not change, even if the liquid level changes. However, if the tops 28C are above the surface of the molten metal, the interface area will change with liquid level. The configuration of the molten metalchamber interface is perhaps the most important factor.

The contact area between the molten metal and each segment of the chamber bottom must be small enough that the critical bubble diameter will not =be reached, while the chamber bottom must have a configuration which will confine the vapor and cause it to follow a path to the liquid surface and escape without the expulsion of liquid particles.

It has been found that graphite is a suitable material for the crucible when zinc is the molten metal. Graphite does not react with the molten zinc, is easily fabricated, is efficient in conductin-g heat from the heating elements 26 to the molten zinc to be vaporized, and is not wetted by the zinc. The grooved configuration provides stable, rapid, and relatively droplet-free production of vapor.

Even though providing a scalloped bottom portion 12a in the crucible 12 substantially eliminates droplets in the vapor, the vapor may not be completely droplet free, especially at rates sufiiciently high for commercial coating purposes. Accordingly, a baflie 30, a filter 32 and a screen 34 for retaining the filter 32 adjacent the bafiie 30 are included. The Ibaliie and filter form a barrier extending parallel to the bottom portion 12a of the crucible so that vapor may not exit from the crucible without passing therethrough.

In the arrangement the metal vapor first fiows through the batiie 30 which defines a tortuous path so that large size droplets are removed from the vaporized metal. The

bafe is formed in three layers, each layer comprising plural parallel elongated strips 36 arranged adjacent each other but with slight spaces left between adjacent strips. The second layer of strips 36 is arranged above the first layer such that each strip of the second layer is disposed above a respective space left between two adjacent strips of the first layer. Similarly, the third layer is disposed above the second layer, each strip of the third layer being disposed above a space left between two adjacent strips of the second layer. Each strip may be curved about its longitudinal axis so that its concave surface 36a faces the bottom portion 12a of the crucible. This curvature serves to trap droplets Within the bafiie 30. The strips 36 may be fabricated from steel tubing that is cut in half longitudinally or from graphite. The strips 36 of the bafiie 30 thus prevent large droplets from passing out of the chamber 12 without prohibiting vapor movement.

Providing a tortuous path for the metal vapor has been found to be somewhat less than totally effective in preventing droplets from being entrained in the vapor. Thus, the lter 32 is located adjacent the baille 30 so as to remove any liquid inclusions remaining in the vapor after passage through the tortuous path. The filter 32 is of a felt material that is non-reactive with the metal vaporized. Where zinc is the metal that is to be evaporated, the felt material preferably comprises a graphite felt. Graphite felt sold under the trademark National by the Carbon Products Division of Union Carbide Corporation has been found to be particularly suitable for the vacuum conditions of the present process. The graphite felt need only be about 1A" thick to accomplish the purpose of preventing the passage of droplets. It has been found that the pressure drop across the graphite felt filter 32 is only about 1 to 5 millimeters Hg when the pressure in the crucible 12 of the metal vapor is about 25 to 50 mm. Hg at a temperature of about 1200-1275 F. As seen in FIGS. 1 and 3, the screen 34 is located adjacent the filter 32, the filter 32 being positioned between the baffle 30 and the screen 34. Support rods or the like (not shown) may be used to hold the screen 34 in place against the filter 32. In FIG. l there is shown vapor Icondensing apparatus 38 provided externally of the metal vaporizing apparatus 10 to receive the metal vapor and deposit it upon a substrate 40 to form a coating thereon. The vapor condensing apparatus 38 does not form part of the present invention and is only illustrated to show the total environment of the invention. The specific vapor condensing apparatus 38 shown in FIG. 1 is the subject of copending application Ser. No. 39,379, filed May 21, 1970, for Apparatus and Method for Continuously Condensing Metal Vapor Upon a Substrate, in the name of Frank I. Cole, and assigned to the assignee of the present application. Preferably, the vapor condensing apparatus 38 comprises a nozzle 42 (through which the metal vapor is directed from the crucible 12 toward the substrate 40) and a condensing chamber 44 communicating with the nozzle 42 and through which the substrate 40 continuously moves. The nozzle 42 and the condensing chamber 44 are maintained at a temperature the same as or in excess of the temperature of the vapor by heating means 46, such as a resistance heater or the like, so that the vapor is deposited upon the substrate 40 rather than upon the walls of the nozzle 42 or the condensing chamber 44. An adjustable valve 48 located in the nozzle 42 controls the flow of the metal vapor by dividing out excess vapor from the ow. A collector box 50 is secured to the nozzle 42 for receiving vapor divided out of the ow by the valve 48. Metal vapor can be deposited in a controlled amount on the continuously moving substrate 40 so as to form a coating thereon.

In practicing the method of the present invention, molten metal is fed through the inlet pipe 18 to the bottom portion 12a of the crucible 12. The molten metal fills the grooves 28 at the bottom of the crucible and is preferably maintained at a level slightly above the tops 2SC of the grooves 28. This results in a constant rate of vaporization as the surface area of the molten metal always remains the same despite evaporation of the molten metal. The liquid level may be maintained just below the tops 28C of the grooves, however. The molten metal is heated by means of heating elements 26 so that metal vapor is generated. The rising vapor from the pool of molten metal may contain droplets despite the configuration of bottom portion 12a which substantially eliminates the development of droplets. To remove the droplets from the vapor, the vapor is passed through bafiie 30 defining a tortuous path and then through the felt filter 32 closely adjacent to the baffle 30. No droplets will be present after passage through the filter 32 up to at least a rate of vaporization of about l lb./sq. ft. of vaporization area/ min.

The filtered vapor is removed from the crucible and passes through the nozzle 42. The vapor then enters the condensing chamber 44 where it is deposited upon a continuously moving substrate 40 so as to form a coating thereon.

EXAMPLE Evaporator data:

Zinc delivered-157 lbs. Zinc in evaporator at time of coating- 140 lbs. Zinc height, time of coating--1z in. above tops 28C. Liquid zinc temperature -1275 F. Vapor temperature-1275 F. Evaporator temperature-1420" F. (bottom) and 1500 F. (top) and 1380 F. (side). Heater element temperature-2640 F. (two outside heaters); 1940 F. (inside heater). Nozzle 42 temperature-1490 F. Condensing chamber 44 temperature-l350 F. Power input- 127 kw. Evaporator data:

Evaporation rate-5.03 lb. per min. Vapor velocity (above felt 32)-10 ft./sec. Vapor velocity (in nozzle 42)-330 ft./sec. Coating data:

Condensation rate-4.79 lb. per min. Strip speed-132 ft. per min. Coating thickness-1.03)( l"3 in. Steel strip dimensionsin. x .0225 in. x 1350 lb.

coil. Coating run duration-10 minutes.

Th'us, the present invention provides apparatus and method for vaporizing molten metal without entrainment of droplets in the metal vapor. The metal vapor may be deposited as a coating upon a fast moving substrate without splatter appearing in the coating. Moreover, the coating is of a uniform thickness on the substrate as the rate of vaporization of the metal is controllable and may be kept constant.

What is claimed is:

1. Apparatus for generating vapor substantially free of liquid droplets from molten metal, comprising:

(a) a crucible for containing a pool of molten metal, said crucible having plural parallel elongated grooves in its bottom portion immersed within the pool and filled with the molten metal, the distance across each groove being less than the diameter of the minimum size molten metal pool necessary to support a bubble nucleus of critical diameter for the particular molten metal involved, the bottom and sides of each groove being rounded, the top of said pool of molten metal being maintained substantially at a level just above said bottom portion;

(b) heating means associated with said crucible for heating said molten metal to generate metal vapor;

(c) bafe means dening a tortuous path through which the vapor flows and by which liquid droplets in the vapor are substantially removed from the generated vapor; and

(d) filter means for filtering the generated vapor in order to remove any splatter remaining in the generated vapor.

2. Apparatus according to claim 1, wherein said filter means comprises a felt material that is non-reactive with the metal vapor.

3. Apparatus for generating vapor substantially free of liquid droplets from molten metal, comprising a crucible for containing a pool of molten metal the bottom portion of said crucible being relatively fiat and having plural grooves therein which are all substantially upwardly directed and which are substantially filled with said molten metal during vaporization, the distance across each groove being less than the diameter of the minimum size molten metal pool necessary to support a bubble nucleus of critical diameter for the particular molten metal involved.

4. Apparatus raccording to claim 3, wherein said grooves are relatively elongated and substantially parallel to each other.

5. Apparatus according to claim 3, including means for maintaining the level of molten metal in said crucible only slightly above the tops of said grooves.

6. Apparatus according to claim 3, including means for maintaining the level of molten metal in said crucible only slightly below the tops of said grooves.

7. Apparatus according to claim 3, wherein said grooves are rounded along their respective bottoms.

8. Apparatus according to claim 7, wherein said grooves are rounded along their respective sides.

9. Apparatus according to claim 3, wherein the grooved bottom portion of the crucible is fabricated from a material that is not wetted by said molten metal.

10. Apparatus according to claim 3, wherein the grooved bottom portion of said crucible is fabricated from graphite when zinc is said molten metal.

11. Apparatus according to claim 1, further comprising a screen disposed adjacent said filter means, said filter means being positioned between said bafiie means and said screen.

12. Apparatus according to claim 1, wherein said baffle means comprises:

(a) a first layer of plural parallel elongated strips arranged so that spaces are left between adjacent strips;

(b) a second layer disposed above said rst layer and of plural parallel elongated strips arranged so that spaces are left between adjacent strips, each strip of said second layer being disposed above a space left between two adjacent strips of said rst layer; and

(c) a third layer disposed above said second layer and of plural parallel elongated strips arranged so that spaces are left between adjacent strips, each strip of said third layer being disposed above a space left between two adjacent strips of said second layer.

13. Apparatus Iaccording to claim 12, wherein said strips are curved about their longitudinal axes and are disposed such that they are concave relative to the bottom of said chamber.

14. Apparatus according to claim 1, wherein said iilter means comprises a layer of graphite felt.

15. A method of generating vapor substantially free of liquid droplets from molten metal, comprising the steps of:

(a) providing a pool of molten metal in a cmcible having plural parallel and upwardly directed elongated grooves in its bottom portion to substantially immerse the grooves and iill them with said molten metal, the distance across each groove being less than the diameter of the minimum size molten metal pool necessary to support a bubble nucleus of critical diameter for the particular molten metal involved, the bottom and sides of each groove being rounded;

(b) heating said molten metal to generate metal vapor;

(o) passing the vapor through a baffle dening a tortuous path by which liquid droplets from the molten metal are substantially removed from the generated vapor; and

(d) ltering said generated vapor to remove any droplets remaining in the generated vapor.

16. A method according to claim 15, further comprising:

(a) removing said vapor from said chamber; and

(b) depositing said vapor on a substrate to form a coating thereon.

17. A method according to claim 15, wherein the level of molten metal is maintained just above the tops of of said grooves.

18. A method acording to claim 1S, wherein the level of molten metal is maintained just below the tops of said grooves.

19. A method according to claim 15, wherein said crucible is maintained under a vacuum.

20. A method according to claim 15, wherein the maximum rate of Vvaporization without any droplets in said vapor is at least l lb./sq. ft. of vaporization area/min.

21. A method according to claim 15, wherein the filtering is accomplished by passing said vapor through a layer of felt material.

22. A method according to claim 21, wherein said felt material is graphite felt.

23. A method according to claim 15, wherein the metal vapor is zinc and lter material to achieve the iiltering is used which is non-reactive with the zinc vapor.

24. A method according to claim 15, wherein the pressure drop during said ltering step is about 1 to 5 mm. Hg when the pressure of the metal vapor is about 25 to 50 mm. Hg.

25. A method of generating vapor substantially free of liquid droplets from molten metal, comprising providing a pool of molten metal in a Crucible, the bottom portion of the Crucible having plural upwardly directed grooves therein which are substantially tilled with molten metal during vaporization, the distance across each groove being less than the diameter of the minimum size molten metal pool necessary to support a bubble nucleus of critical diameter for the particular molten metal involved, and heating the pool of metal to vaporize it.

26. A method according to claim 25, including maintaining the level of molten metal in the chamber only slightly above the tops of said grooves.

27. A method according to claim 25, including maintaining the level of molten metal in the chamber only slightly below the tops of said grooves.

28. A method according to claim 25, in which the pool of molten metal is provided in a crucible in which the distance across each groove thereof is less than the critical diameter of vapor bubbles of the molten metal.

References Cited UNITED STATES PATENTS 3,055,775 9/1962 Crittenden 117-107 3,330,647 7/ 1967 Hunt 75-93 3,282,797 11/ 1966 Hammer 202-236 2,881,116 4/1959 Siegfried 202-197 2,842,224 7/ 1958 Mooradian 202-l97 3,427,227 2/ 1969 Chamberlin 202-197 3,457,142 7/ 1969 Starmer 203-40 1,994,349 3/1935 Ginder 75-88 2,983,493 5/ 1961 Handwerk 266-34 2,668,046 2/ 1954 Robson 266-15 2,939,783 6/1960 Lundevall 266-17 3,016,237 l/l962 Caron 266-Il5 L. DEWAYNE RUTLEDGE, Primary Examiner P. D. ROSENBERG, Assistant Examiner U.S. C1. X.R. 

